The present invention relates to a telephoto lens system, and, more particularly, to a telephoto lens system for use in a 35-mm single-lens reflex camera that insures high performance despite its compact size, and which performs focusing by moving only one lens unit in the system.
With the advent of cameras containing automatic focusing mechanisms, the advantages of inner focusing telephoto lenses increasingly has been recognized. A major requirement for lenses adapted for automatic focusing is that the movable lens units be sufficiently lightweight to achieve focusing without burdening the lens drive source. In particular, regarding telephoto lenses which theoretically cannot be manufactured without increasing the optical system size, an inner focusing feature has become essential to achieve automatic focusing.
Accordingly, various designs of inner focusing telephoto lens systems have been developed to take advantage of their features of "light" focusing, small centroid movement, and stability during focusing. Some of the designs proposed to date are described in JP-A-59-214008 (the term "JP-A" as used hereunder means an "unexamined published Japanese patent application"), JP-A-59-170811, and Japanese Patent Application No. 62-26787 which was filed previously by the common assignee.
Despite the advantages described above, inner focusing lens systems generally have several problems. For example, in order for focusing units to move within the optical system, often the overall lens system size must be increased. If the refractive power of the focusing groups is increased to reduce the amount of their movement, great aberrational changes will result during focusing at near distance. Accordingly, designing a compact inner focusing telephoto lens system has been difficult, as compared with designs that perform focusing by advancing not only the front lens, but also the overall system. The system described in JP-A-59-214008 features a telephoto ratio of about 0.8, and the closest focusing distance is 7.5f (where f is the overall focal length) when inner focusing is adopted.
In contrast, JP-A-59-170811 and Japanese Patent Application No. 62-26787 propose compact systems that have comparatively large values of F.sub.NO and small telephoto ratios. However, they suffer from the closest focusing distance being long, great aberrational variations occurring when focusing, and the powers of individual lenses being so strong that the system performance varies due to lens manufacturing inconsistencies.
The telephoto lens system described in JP-A-59-170811 has a telephoto ratio of 0.71 and a closest focusing distance of 10f. A negative meniscus lens is positioned just behind the first lens group and by making it partly responsible for compensation for spherical aberration, the burden on the focusing lens units of compensating for spherical aberration is reduced sufficiently to ensure efficient compensation for aberrational changes that will occur during focusing. However, if further reduction of the overall size of the system by this method is attempted so that spherical aberration can be compensated for satisfactorily by the meniscus lens, the Petzval sum oftentimes will assume a negative value to cause either curvature of the field, or astigmatism. In other words, the telephoto ratio of the system proposed in JP-A-59-170811 cannot be made smaller without causing residual astigmatism, or insufficient compensation for the variations in spherical aberration.
The telephoto lens system described in Japanese Patent Application No. 62-26787 has a telephoto ratio of 0.68 and a closest focusing distance of 7f. The second lens group in this system is a positive lens unit, whereas the third lens unit is composed of a cemented negative lens unit and a positive lens. The advantages, if any, of employing the positive second lens unit are not clear since they are not stated expressly in the specification. Regarding the composition of the third lens unit, if the positive first unit (III-1) in the third lens unit is cemented to the negative second unit (III-2), the refractive index of the positive first unit must be increased and negative spherical aberration must be developed at the cemented surface in order to compensate for the spherical aberration that might occur in the third lens unit. In other words, the Petzval sum is reduced to the negative side by this arrangement, thus being liable to the development of field curvature. In order to solve this problem, Japanese Patent Application No. 62-26787 makes the following proposal: in order to compensate for the negative Petzval effect of the third lens unit, the individual lens surfaces of the first lens unit are designed to have a very strong power so that the Petzval sum is shifted to the positive side to ensure that aberrations are compensated for effectively. However, from a manufacturing viewpoint, lens systems that adopt the method of aberrational compensation described above are very difficult to fabricate because the absolute values of the third-order aberration coefficient, which measures the effect that might be caused on the surface precision aberration, are particularly great at the first, fifth, sixth, seventh and tenth surfaces. This will inevitably lead to variations in lens performance due to non-uniformity in the machining precision.